Apparatus and method for detecting arcing in a CRT

ABSTRACT

A method and apparatus for detecting and indicating the number of times a CRT unit arcs. The apparatus is constructed of a ferrite split toroid core wound with wire which is coupled around the focus lead and/or ground lead of a CRT through which a current spike will occur during an arcing event. The wound wire is coupled to a current pulse detector which in turn is coupled to a monostable mode timer and counter for counting the number of arcing events. The counter is coupled to a display device for displaying the number of times the CRT has arced.

FIELD OF THE INVENTION

The invention pertains to the manufacture, testing and quality assuranceof cathode ray tubes (CRTs). More particularly, the invention pertainsto a method and apparatus for counting the number of times a CRT arcs.

BACKGROUND OF THE INVENTION

As is well known to those skilled in the related arts, one of theprimary components of a cathode ray tube (CRT) display apparatus is theelectron gun or guns. In particular, a visual display is created on aphosphorescent screen by a CRT by scanning one or more electron beamsacross the screen to selectively illuminate picture elements (pixels) onthe screen to create a desired display pattern. A monochrome CRTtypically comprises a single electron gun. A color CRT typicallycomprises three electron guns (corresponding to the colors red, greenand blue). The combination of the three primary colors in differentratios on any given pixel can create any color in the visible spectrum.

Typically CRTs are manufactured in a clean room environment. Even theslightest contamination by dust or other particles in the neck assembly(which houses the electron gun and the space through which the electronbeam or beams travel from the gun to the screen) may significantlyaffect operation of the CRT. Particularly, if a dust particle or otherparticle is struck by an electron beam, it typically would cause the CRTto arc, resulting in a current surge in the CRT. The particular lead orleads on which the current surge would occur depends on a number offactors, such as the particular electron beam which struck the particle.The surge, however, would most likely occur on the focus leads and/orground leads of an electron gun. If the current surge is great enough,it can burn out one or more of the electronic components of the CRT.

Even if an arcing event does not destroy or harm electronic components,it can cause the calibration of the electron gun or guns to be changed.For instance, in a color monitor, the three electron guns are calibratedrelative to each other to create the desired color scheme. If these gunsare calibrated while a dust particle is in the electron path of one ofthe guns, they will be calibrated to certain values relative to eachother. If a dust particle which existed in the electron path when theinitial calibrations were made is later zapped (burned by the electronbeam), the initial calibrations may no longer be valid. For instance, ifthe red electron gun was adjusted to a nominal voltage of 40.2 volts inorder to create the desired color scheme, and a dust particle in itselectron beam path is later zapped, the 40.2 volts adjustment may nolonger be valid and the display may become skewed toward the red end ofthe color spectrum.

Arcing in the field, leading to operational failure of the CRT or atleast to color skew, is one of the more common field failure mechanismsin CRTs.

Most CRT manufactures have quality assurance tests which test the CRTfor many possible defects including the existence of particles in theelectron beam path which cause or may cause arcing or excessive arcing.It is known that as the number of arcing events increases duringmanufacturing and/or testing, the more likely the CRT unit is tocontinue arcing in the field. Excessive arcing during testing indicatesthat the CRT was either manufactured under less than perfectly cleanconditions or that its hardware is itself defective. In either event, itsuggests that arcing will continue to occur in the field.

If the arcing was due to dust, as opposed to hardware defects, arcing islikely to continue to occur because it is possible and even likely thatother dust particles exist in the neck assembly and that they will moveduring transportation, thus not causing arcing during testing, but onlyafter the unit is sold and in operation in the field.

Accordingly, it is desirable to determine how often a CRT arcs duringthe manufacturing and/or quality assurance testing of the unit. Althoughduring manufacturing and testing, a CRT unit may be powered up andoperating for several hours (during which time arcing may occur), mostof this time is spent outside of the presence of a person who canactually observe the occurrence of arcing. For instance, duringmanufacturing, a CRT is typically "burned" in an aging tunnel for atleast an hour. In the burning process, a CRT unit is powered up andoperated in a high temperature environment in order to stabilize itscomponents, particularly the aperture grill. The aperture grill affectsthe alignment of the beams and typically does not stabilize until atleast thirty to forty-five minutes of operation in an aging tunnel. Theburning stage is normally conducted outside of the observation ofhumans.

Accordingly, it is an object of the present invention to provide amethod and apparatus for detecting and counting arcing events of a CRT.

It is another object of the present invention to provide a low power,low cost, and digital method and apparatus for detecting arcing in aCRT.

It is a further object of the present invention to provide an improvedCRT arc detector.

SUMMARY OF THE INVENTION

The invention comprises a palm size arc detector having one or moreprobes formed of a split ferrite toroid core wound with ten turn wire.Each probe can be coupled around a lead of a CRT. The lead or leads towhich the probe or probes are coupled should be those leads throughwhich a current spike is expected to appear during arcing. Typically,this would include at the least the focus leads and the ground leads ofthe CRT. The electromagnetic field generated by the current spike in theselected leads will cause a corresponding current spike to run throughthe ten turn wire of the probe. The wire of each probe is coupled to acurrent pulse detector which triggers a monostable mode timer every timea current pulse of a predetermined magnitude (e.g., a magnitude highenough to have been caused by an arcing event as opposed to normaloperation of the CRT) runs through the probe wire. The magnitude of acurrent spike caused by an arcing event is substantially higher than themagnitude of the current which runs through the sensed leads duringnormal operation of the CRT and, therefore, can be easily differentiatedtherefrom.

The output of the timer is fed to a counter which counts the number ofarcing events. The counter is coupled to a display which displays thenumber of arcing events detected.

If the number of arcing events detected exceeds a predetermined numberover a predetermined time period (e.g., if it exceeds two events duringthe burning stage), the unit is rejected as being likely to continue toarc in the field.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a block diagram of the major components of the arc detector ofa preferred embodiment of the present invention.

FIG. 2 is a detailed schematic diagram of a preferred embodiment of thepresent invention.

FIG. 3 is a pictorial diagram of an arc counter constructed inaccordance with a preferred embodiment of the present invention.

DETAILED DESCRIPTION OF A PREFERRED EMBODIMENT OF THE INVENTION

FIG. 1 is a block diagram of an arc counter 10 in accordance with thepresent invention. A nine volt battery 12 provides the power foroperating the unit. An on/off switch 14 also is provided. An inductor 16is formed by ten turns number 26 enameled wire 18 wound on half of asplit toroid core 20. The lead 19 of a CRT on which an arcing event willcause a current spike is placed in the middle 17 of the toroid core 20.The toroid is split and hinged at one end of the split to allow it to beopened so that the lead 19 can be inserted and the toroid 20 closedagain.

FIG. 1 shows only one inductor probe 11. However, as many probes can beprovided and coupled in parallel to the main circuitry of the arcdetector as there are different leads through which an arcing event canbe manifested as a current spike. The wire 18 wound around the core iscoupled to a current pulse detector 24. Current pulse detector 24 isdesigned to send a pulse out on line 26 to trigger pulse circuit 28whenever the current through wire 18 exceeds a specified level. Thespecified current level is set to a level less than the minimum currentspike expected to be caused in wire 18 in response to an arcing eventcurrent spike in lead 19, but greater than the maximum current whichshould be caused to flow through wire 18 when sensing normal CRToperation current levels on lead 19. The current flow in wire 18 shouldbe essentially equal to the current flow in the sensed lead 19 whenplaced in the middle 17 of the toroid 20. Thus, if current flow throughthe focus leads and/or ground leads of a CRT during normal operationwithout arcing is expected to be much less than 1.0 ampere and thecurrent surge during an arcing even is expected to be over 1.3 amperes,the reference current of the counter reasonably may be set toapproximately 1.3 amperes.

Trigger pulse circuit 28 sends out a pulse on line 30 to counter 32 eachtime it receives a current pulse detection signal on line 26.

Counter 32 preferably is a BCD counter which increments its multi-bitBCD output lines 34 each time it receives a pulse on line 30.

The BCD output 34 is forwarded to a display unit 36 which converts theBCD output 34 into a human readable form indicating the number of arcingevents which have occurred.

In a preferred embodiment of the invention, the display device 36comprises a plurality of LEDs which are coupled so as to be drivendirectly by the BCD output lines. Although it may be more convenient tothe user to have a more complex display system in which the BCD outputlines are converted into a LED or LCD numeric display, the abovedescribed display system is preferred for its low power consumption andcircuitry requirements.

After each CRT is tested, the counter 32 and display 36 are reset andthe arc counter can be coupled to another CRT in order to performanother test.

FIG. 2 is a detailed schematic diagram of an arc counter 10 inaccordance with the present invention. As shown, this particular unitcomprises two probes 11a and 11b. Accordingly, probe 11a can be coupledto a focus lead of a CRT while probe 11b is coupled to a ground lead ofa CRT. Accordingly, arcing events which cause current spikes at eitherthe focus lead or ground lead will be detected. As will become clear, inthis particular embodiment of the invention, there is no way todistinguish whether a counted arcing event occurred on the focus lead orthe ground lead. Such information typically will not be relevant anyway,since the CRT unit will simply be rejected if the number of arcingevents exceeds a predetermined number regardless of which leads thoseevents occurred on.

If it was desirable to distinguish between the leads upon which acurrent spike occurred, the unit could easily be redesigned to do so,such modifications being obvious to a person of ordinary skill in therelated arts. For example, each probe could be coupled to separatedetection, counting and display circuitry.

Current through the lead to which probe 11a is coupled will causecurrent to flow through the wire 18a. That current causes a voltage toappear across resistor R1. As shown, resistor R1 is a variable resistorwith on of its terminals coupled to the base of transistor Q1 so thatthe portion of that voltage drop which appears at the base of transistorQ1 is adjustable. Capacitors C3 and C5 split the voltage drop,preferably in half. Zener diode ZD1 is provided to protect the circuitryof the arc counter 10 from extremely high current surges and, in apreferred embodiment, is a 6.2 volt zener diode. The resistor R1 isadjusted such that the desired predetermined minimum current creates avoltage at the top of resistor R1 just great enough to forward biastransistor Q1.

When a current greater than or equal to the predetermined minimumcurrent flows in wire 18 due to a current surge on the sensed leads ofthe CRT, it turns on transistor Q1 of current detector 24a. Thecollector of transistor Q1 is coupled to the trigger terminal of amonostable mode timer 44 of circuit 28. The voltage at the collector oftransistor Q1 will go low when transistor Q1 is activated thustriggering low activated monostable mode timer 44. When triggered, timer44 outputs a 10 millisecond pulse to the CLK input pin of BCD up counter46 of counter circuit 32. Each time BCD up counter 46 receives a pulseat its clock input terminal, it increments its BCD output terminals Q0through Q3.

As shown, BCD output lines D0 through D3 of counter 46 are coupled tothe base terminals of transistors Q4 through Q7, respectively. Thecollectors of those transistors are coupled to four LEDs, LED1 throughLED4, causing the LEDs to be turned on when the respective BCD outputline goes high, and to be turned off when the respective BCD output linegoes low. Accordingly, the four LEDs constitute a BCD display on thepanel of the arc counter unit 10.

The control panel of the unit 10 is shown in FIG. 3. In addition to thefour LED BCD display 60, the unit 10 has only two operator usableswitches. The first switch 62 is an on/off switch which can be toggledbetween on and off, effectively connecting or disconnecting the battery.The second switch 64 is a three position switch which can be toggledbetween three positions, namely, a reset position, a neutral position,and a test position. The switch 64 is permanently biased to the neutralposition. A user can overcome the biasing pressure with hand pressure toplace the switch in the reset position or the test position. However,when the hand pressure is released, the switch will return to theneutral position.

When in the neutral position, test switch SW1a and reset switch SW1bshown in FIG. 2 are both open circuited. When in the reset position,reset switch SW1b is closed and test switch SW1a is opened. Finally, inthe test position, test switch SW1a and reset switch SW1b is opened.

As can be seen from a review of the circuit in FIG. 2, when the resetswitch SW1b is closed, the reset input terminal of counter 46 isactivated to reset the counter. When the test switch is closed, the baseof transistor Q1 is activated thus causing timer 44 to be triggered.

Accordingly, in operation, a user typically would toggle the secondswitch to the test position one or more times and observes the displayto assure that the display counts upward for each time the second switchis toggled to the test position. If it does not increase each time, thenthe arc counter is not operating properly. After testing the counter,the user then toggles the second switch to the reset position to resetthe counter and display to zero before commencing arc counting on a CRT.

It has been observed that if a CRT which arcs two or more times duringthe burning stage is substantially more likely to continue arcing thanis a CRT which either does not arc or arcs only a single time during theburning stage. Accordingly, if the arc counter displays a count of twoor more arcing events during the burning stage, that CRT typically wouldbe rejected.

As noted in FIG. 2, the display unit is limited to a count of eight. Thereason for this is simply that, typically, if a CRT arcs eight or moretimes, it clearly should be rejected regardless of whether the number ofarcing events was eight, nine, ten, etc. Accordingly, it not necessaryto count any higher than eight.

Probe 11b and current pulse detector 24b shown in FIG. 2 aresubstantially identically to probe 11a and current pulse detector 24aand are coupled essentially in parallel with probe 11a and current pulsedetector 24a to the trigger input terminal of timer 44. If there aremore than two leads which should be tested on a given CRT, additionalprobes and current pulse detectors can be added and coupled in parallelto the trigger input of timer 44.

Having thus described a few particular embodiments of the invention,various alterations, modifications, and improvements will readily occurto those skilled in the art. Such alterations, modifications andimprovements as are made obvious by this disclosure are intended to bepart of this description though not expressly stated herein, and areintended to be within the spirit and scope of the invention.Accordingly, the foregoing description is by way of example only, andnot limiting. The invention is limited only as defined in the followingclaims and equivalents thereto.

I claim:
 1. An apparatus for detecting and counting arcing events of acathode ray tube (CRT) comprising;an inductor for electromagneticcoupling to a lead of a CRT, a current detector coupled to said inductorfor generating a current pulse signal when a current exceeding aspecified value flows through said inductor, and a counter for countingthe number of current pulse signals generated by said current detector,and a display for displaying the number of current pulse signalsgenerated by said counter.
 2. An apparatus as set forth in claim 1wherein said inductor comprises a wire wound around a split toroidferrite core.
 3. An apparatus as set forth in claim 1 wherein saidcounter comprises a binary coded decimal (BCD) counter having aplurality of output lines for representing the count of said counter inBCD and said display comprises a plurality of light emitting diodescoupled to said output lines of said counter.
 4. An apparatus as setforth in claim 1 wherein said current detector comprises;a transistorcoupled to said inductor so as to be turned on when said current exceedssaid specified value, and a monostable mode timer having an inputcoupled to said transistor for generating an output signal comprising apulse each time said transistor is turned on.
 5. An apparatus as setforth in claim 4 wherein said counter comprises a BCD counter having aninput coupled to said output signal of said timer for counting up eachtime it receives a pulse from said timer.
 6. An apparatus as set forthin claim 4 wherein said inductor comprises a plurality of inductors andsaid current detector comprises a current detector corresponding to eachof said inductors and said current detectors are coupled in parallel tosaid timer.
 7. An apparatus as set forth in claim 4 wherein saidtransistor comprises a base terminal, a collector terminal and a drainterminal and wherein said base terminal is coupled to said inductor andsaid collector is coupled to said timer and wherein said transistor isswitched on responsive to said current exceeding said specified value soas to cause current to flow through said transistor from said collectorterminal to said drain terminal causing a voltage change at said inputterminal of said timer.
 8. An apparatus as set forth in claim 1 furthercomprising a reset switch for resetting said counter to zero.
 9. Anapparatus as set forth in claim 8 further comprising a reset switchcoupled to said counter for resetting said counter to zero.
 10. Anapparatus as set forth in claim 1 further comprising a test switchcoupled to said counter for causing said counter to count up each timesaid switch is closed, whereby proper operation of said counter can betested.
 11. An apparatus as set forth in claim 7 further comprising atest switch coupled to said base terminal of said transistor whichswitch, when closed, causes a current to flow at said base terminal ofsaid transistor regardless of the condition of said CRT, whereby properoperation of said timer, BCD counter and display can be verified.
 12. Anapparatus for detecting and counting arcing events of a cathode ray tube(CRT) comprising;an inductor for electromagnetic coupling to a lead of aCRT so as to cause a current to flow in said inductor when a currentflows in said lead of said CRT, a transistor having a base, a collectorand a drain, said base coupled to said inductor so as to cause a currentto flow at said base of said transistor when said current in saidinductor exceeds a specified value such that said transistor is turnedon and a current flows from said collector to said drain of saidtransistor, a monostable mode timer having an input coupled to saidcollector of said transistor, said timer generating an output signalcomprising a pulse each time said transistor is turned on, a BCD counterhaving an input coupled to said output signal of said timer and aplurality of output lines, said counter causing said output lines tooutput a BCD signal indicating the number of counts of said counter, anda display comprising a plurality of light emitting diodes, each coupledto one of said output lines of said counter so as to be activatedthereby, whereby a BCD visual display of the count of said counter iscreated.
 13. An apparatus as set forth in claim 12 wherein said inductorcomprises a wire wound around a split toroid ferrite core.
 14. Anapparatus as set forth in claim 13 wherein said inductor comprises aplurality of inductors and said transistor comprises a transistorcorresponding to each of said inductors and said plurality oftransistors are coupled in parallel to said timer.